1. Field of the Invention
The invention relates generally to thermally activated, self-releasing seals, valves, plugs and the like (hereinafter referred to as seals), used to retain a fluid within a containment vessel (sometimes referred to hereinafter as a "thermal mass container") until such time as the fluid stored in the vessel is heated to a predetermined temperature. Once the predetermined temperature is reached, a thermally activated self-releasing seal will release (open), allowing the fluid in the containment vessel to escape.
The invention more particularly relates to a thermally activated self-releasing seal that may, for example, be used as a boiler plug for a boiler (which is an example of thermal mass container) incorporated into aircraft flight recorders. An example of such a boiler is described in copending patent application serial number (Attorney Docket LRL-001), entitled "Flight Crash Survivable Storage Unit With Boiler For Flight Recorder Memory", filed on Oct. 5, 1994, and assigned to the same assignee as the instant invention.
The present invention specifically relates to methods and apparatus for sealing an aperture in a thermal mass container using a one piece, thermally activated, self-releasing seal which is affixed to the exterior of the container using, for example, an adhesive to actually create the desired seal by releasably securing the one piece structure in place.
The seal contemplated by the invention is formed as a combination of elements, namely: (a) a seal support means which is located on the exterior of, and is "separate and detached" from the container (where "separate and detached" is defined herein to mean not directly attached to the container at any time); (b) a fusible seal which is attached to the seal support means to form the one piece seal when combined with the seal support means (the fusible seal material, according to a preferred embodiment of the invention, being in a non-fused state after the one piece seal is formed); and (c) means for attaching the fusible seal portion of the one piece seal to the container (using, for example, the aforementioned adhesive), to cover an aperture in the container, wherein the means for attaching is located on the exterior of and is separate from the container.
The invention also relates to (a) processes for manufacturing sealed fluid containment vessels having a thermally activated safety seal; and (b) processes for fabricating such seals in circumstances where the materials used for the seal and containment vessel are incompatible for bonding purposes.
2. Description of the Related Art
The prior art is replete with patents directed to self-releasing sealing mechanisms which are thermally activated.
Examples include U.S. Pat. No. 1,973,182, to Shaw, entitled "Heat Releasable Drainage Means", issued Sep. 11, 1934; U.S. Pat. No. 3,168,210, to Williams, entitled "Safety Feature In Pressurized Containers", issued Feb. 2, 1965; U.S. Pat. No. 4,147,272, to Stenner et al., entitled "Pressure Relief Device", issued Apr. 3, 1979; U.S. Pat. No. 4,195,745, to Roberts et al., entitled "Thermally Sensitive Pressure Release Assembly For Sealed Pressurized Vessel", issued Apr. 1, 1980; U.S. Pat. No. 4,232,796, to Hudson, Jr., et al., entitled "Thermal Release Plug For A Fabricated Pressure Vessel", issued Nov. 11, 1980; U.S. Pat. No. 4,690,295, to Wills, entitled "Pressure Container With Thermoplastic Fusible Plug", issued Sep. 1, 1987; U.S. Pat. No. 4,744,383, to Visnic et al., entitled "Thermally Activated Valve", issued May 17, 1988; U.S. Pat. No. 4,899,777, to Stone et al., entitled "Thermally Activated Pressure Relief Plug", issued Feb. 13, 1990; U.S. Pat. No. 5,154,201, to Yanagihara et al. entitled "Method Of Manufacturing A Sealed Vessel Having A Safety Valve", issued Oct. 13, 1992; and U.S. Pat. No. 5,193,046, to Lemke et al., entitled "Information Recording Apparatus With A Liquid Bearing", issued Mar. 9, 1993.
The Shaw patent is directed to heat releasable drainage means for automobile gasoline tanks. Shaw teaches the use of a fusible plug (a seal) installed with a threaded nipple into a wall of the liquid container (a gasoline tank). The seal is relatively thick and the container wall is likewise thick enough to accommodate the installation of the plug.
Shaw typifies the teachings of many prior art references that are directed to installing a plug, seal, valve, etc., within the wall of the containment vehicle, generally referred to hereinafter as "internal seals".
In addition to requiring threading to attach his fusible drain plugs (seals) to the container, Shaw separates his fusible material from the flammable contents of the gas tank by use of a sheet metal disc for safety purposes.
It would be desirable, particularly in view of the teachings of Shaw, to provide less complicated thermally activated self-releasing sealing mechanisms, i.e., provide methods and apparatus for sealing an aperture in a thermal mass container which do not require internal installation within the container wall, thread fabrication and engagement, thick container wall structures provided only for the sake of containing the desired seal, devices to separate the fusible seal from the containers fluid contents, etc.
The Williams patent is generally directed to safety features for pressurized containers such as the type used to store and dispense various substances such as insecticides, deodorizing compounds and the like. In particular, Williams teaches the use of a soft solder fusible seal, attached in a fused state directly to the outside of a pressurized container, which is thermally released when the container reaches a predetermined temperature.
The Williams patent typifies an "external seal", which is the same type of seal contemplated by the present invention. An external seal is defined herein to be a seal affixed to the outside a fluid containment vehicle; as opposed to the aforementioned internal seals which penetrate or are installed with a wall of the containment vehicle.
Williams' teaching of attaching the fusible seal in a fused state, directly to the outside of a pressurized container, can be problematic in many applications. For example, if the containment vessel is first filled using the vent hole being sealed (which is the case in, for example, the flight recorder application referred to hereinbefore in order to minimize the number of container openings, reduce manufacturing costs, conserve save space, etc.), undesirable fluid loss may occur from the heating that takes place when the fusible seal is directly attached to the container in a fused state.
Furthermore, where the fluids contained in the containment vessel are volatile (e.g., gasoline, etc.), the heating required to affix the external seal in the manner contemplated by Williams would be dangerous.
Accordingly, it would be desirable to provide an alternative external thermally activated self-releasing sealing mechanism that includes a fusible material that may be attached to the containment vessel in a non-fused state for the reasons set forth hereinabove.
Visnic et al., U.S. Pat. No. 4,744,383, issued May 17, 1988, is directed to a thermally activated valve which is physically attached to a protrusion on the container being sealed. The valve includes and supports a fusible material which melts and thereby opens the valve at a predetermined temperature. The fusible seal itself is not directly attached to the container in any fashion, being retained by the valve structure which supports the fusible seal.
Since the mechanism for supporting Visnic et al's. fusible seal is itself directly attached to (or alternatively within) a container protrusion, it would be difficult, if not impossible, for the Visnic et al. valve to be used in conjunction with or on "smooth walled" containers. As used herein the term "smooth walled" container means a container not having any protrusions (or recesses) onto which (or within which) a seal included in a valve (such as the valve described by Visnic et al.) may be affixed. An example of a smooth walled container is the container used in the fabrication of aforementioned flight recorder boiler described in the incorporated copending patent application.
It may also be seen that the Visnic et al. valve is a combination of machined pieces that are relatively complex to integrate and manufacture.
Accordingly, it would be desirable to provide a relatively simple, low cost, one piece design, thermally activated self-releasing sealing mechanism that is suitable for use on smooth walled containment vessels, such as the boilers used in the aforementioned flight recorder application.
The aforementioned patents to Stenner et al.; Roberts et al.; Hudson, Jr. et al.; Wills; Stone and Lemke et al., are all further examples of the above described "internal" type seal (such as the one taught by Shaw).
In particular, (1) Stenner et al. simply shows the use of a solder plug that is located within a container wall; (2) Roberts et al. teaches a pressure relief assembly that is thermally sensitive; but does not use a fusible seal for activation purposes; (3) Hudson Jr., et al. teaches the use of a steel ball trapped within a well formed in the wall of a fluid containment vessel, to serve as a seal; (4) Wills teaches the use of a thermoplastic fusible plug within the walls of a container for providing pressure relief; (5) Stone teaches the use of a thermally activated pressure relief plug for mounting within a port on a pressurized structure; and (6) Lemke et al. teaches the use of an internal type seal in an information recording device context (in particular, a flight recorder), where the sealing means (located within a chamber wall) is a solder type seal which melts upon exposure to heat allowing a fluid stored within the chamber to vent.
All of these patents exemplify the state of the art, particularly the art of using internal type seals and seals used in the flight recorder context; and further highlight the need for a simple, external type, thermally activated self-releasing seal which is suitable for use in conjunction with smooth walled containers, suitable for use in environments where space limitations are critical, etc.
Finally, the patent to Yanagihara et al. further exemplifies the state of the art with respect to external type seals (like the seal taught by Williams discussed hereinbefore). In particular, Yanagihara et al. describes a container which has a cavity, located in a concave surface of a containment vessel, for retaining a fusible material which serves as a thermally activated self-releasing seal. The fusible material is directly attached to (i.e., not separate from) the container wall (like the seal taught in the Williams patent) and must first be melted to form the attachment directly thereto. Accordingly, the same improvements desired with respect to the sealing mechanism taught by Williams are desirable with respect to the teachings of Yanagihara et al.
In view of the present state of the art relating to thermally activated self-releasing seals, as exemplified by the patents discussed hereinabove, it would be desirable to be able to provide a new type of thermally activated self-releasing seal exhibiting the desirable properties set forth hereinbefore.